The invention relates to techniques for manipulating digitally compressed and coded data (for example video information) in order to convert it from one format (or specification) to another. Such a method will be referred to herein as digital transcoding, and a device with such a functionality will be referred to as a digital transcoder.
The digital video compression standard developed by International Standardization Organization's (ISO) Moving Picture Expert Group (MPEG) is becoming a key technology in the delivery of digital video programs over a wide variety of media such as terrestrial broadcasting, telecommunication, and cable. It is now almost certain that a digital HDTV (high definition television) standard similar or compatible with standards recommended by MPEG will be used for terrestrial HDTV transmission in North America and Europe. Similar technology will also be used to provide HDTV and standard definition television (SDTV) over cable, phone, fiber optic, satellite and ISDN networks.
U.S. Pat. No. 5,243,428 discusses the MPEG standard as well as its block and frame coding protocols. This patent is incorporated by reference herein. Further details about inter/intra frame and MPEG like video coding can also be found in the following references which are also incorporated by reference herein:
MPEG: A Video Compression Standard For Multimedia Applications; Le Gall, Communications of the ACM, Vol.34, No. 4, April, 1991.
Advanced Digital Communications, Feher, Prentice-Hall Inc., Englewood Cliffs, N.J. (1987);
The Digital Simulcast AD-HDTV Coding System, IEEE Transactions on Consumer Electronics, Vol. 38, No. 4, November, 1992;
Information Technology-Generic Coding of Moving Pictures and Associated Audio, ISO/IEC committee draft, November 1993; and
Test Model 5. Draft-Test Model Editing Committee, ISO/IEC, April, 1993.
SDTV is defined herein as a digitally encoded television signal which can deliver a television picture comparable in overall format and resolution to conventional (e.g. NTSC or PAL) type television pictures. Using techniques for coding HDTV source signals into conventional television channel bandwidths (e.g. 6 MHz.), several SDTV programs can be provided on each channel instead of a single HDTV program.
Introduction of HDTV will probably begin before receivers capable of reproducing the full HDTV source signal are either available or affordable by most viewers. There will therefore be a need to convert HDTV signals to SDTV signals (i.e. transcode them) so that they can be further processed for display on conventional (e.g. NTSC) television receivers which will only be able to decode and display standard definition video.
A digital transcoder may be located at an intermediate stage in the transmitting chain or as part of a telecommunication network such as at a head-end or at a network switch. As currently envisioned, a transcoder will receive the HDTV signal(s) from a central location via satellite or other network communications link and transcode one or more SDTV signals from respective HDTV signals. Both HDTV and SDTV signals will then be transmitted to the home.
Although initially the transcoding equipment will be placed in the transmission chain requiring channels to be provided for both HDTV and SDTV signals, low cost ICs will eventually become available to enable the transcoder to migrate to the consumer's home. The advantage of having the transcoder in receiving chain is that only the HDTV signal will actually have to be transmitted (rather than simulcast with the SDTV signal) and channels occupied by the SDTV signals will be utilized for other uses.
Presently, transcoding from HDTV to SDTV is accomplished by completely decoding the HDTV signal to form a sequence of high definition images (“HD image sequence”). The HD image sequence must then be filtered and subsampled to extract a sequence of lower definition images (“SD image sequence”). The SD image sequence must then be processed to compute SD macroblock information, for example macroblock type information, motion vector information and quantizer information, in order to encode it. However, as is the case with most broadcast quality video systems, a complete encoder is expensive and it therefore would not be practical to include one in a cost effective transcoder designed to be used in the receiving (or transmitting) chain.
An object of the instant invention is, therefore, to provide a method and apparatus for performing cost effective transcoding which avoids having to compute SD macroblock information from the SD image sequence.